1. Field of the Invention
This invention relates to circuitry for driving an inductor, such as a magnetic antenna, which achieves phase modulation with only minimal losses of power needed to overcome losses in the inductor itself.
2. Description of Prior Art
When consideration is given to the roll of new technology in lowering the cost of finding and producing oil and gas, improvements in directional drilling are regularly recognized as one of the important driving factors. The number of successful applications of precisely directed and/or horizontally drilled wells for off shore development and in fractured or compartmentalized reservoirs has been growing rapidly over the past decade. However, if the potential benefits of directionally placed well bores are to be captured for a wider number of reservoirs, particularly those on shore and with more marginal economics, additional cost reduction in directional drilling technology is required.
One opportunity is to reduce the cost and increase the reliability of transmitting downhole sensor information to the surface, a key element of any directional drilling system. Presently, there are two methods of transmission and use: wireline steering tools and measurement-while-drilling (MWD) through mud pulse telemetry.
Wireline steering tools and MWD methods are used in the majority of holes drilled today. Wireline steering tools, which are considered to be the most reliable, transmit both data and power over a hard wired electrical connection between the downhole sensors and the surface. The disadvantages are related to the need for a wireline truck or skid and the necessity of using wet connections. Wet connects are required to access the sensor-stored information on a regular basis, but can be unreliable in certain situations, particularly when the enclosing fluid is highly conductive. There is also an accompanying increase in the number and cost of personnel required to operate these systems compared to an MWD system.
MWD systems rely on producing a positive or negative downhole pressure change in the fluid flowing through the drill pipe. The pulse amplitude at the surface is typically 100 to 200 pounds per square inch (psi) and is a pressure change above a static pressure that may be several thousand psi. There are also pressure variations introduced by the triplex mud pumps commonly in use. MWD systems are by nature slow, with bit rates of less than 1 bit per second (bps), and for reliable operation, fluid flow must be controlled tightly. If there are lost circulation problems, the MWD system usually fails to perform. Some MWD systems work well in vertical holes but fail in horizontal applications.
The ideal downhole data transmission system would be portable, easy to operate, reliable over a wide range of situations, and relatively low cost. One possible contender for this spot would be an electric dipole system. This approach is based upon applying a voltage difference between two sections of isolated drillstring.
Because a typical drillstring might be 100 times the length of the dipole end sections, physics determines that each volt applied by the transmitter should result in a microvolt at the receiver. In fact, the received signal is even stronger because the transmitter is not really a dipole but has a long conducting element (the drillstring), leading all the way to the surface.
Unfortunately, the received signal is much weaker and can become undetectable when conducting upper layers short out the signal. In addition, such a system will not operate inside a cased hole for the same reason. These factors make the electric dipole less attractive as an alternative transmission system.
Phase modulation is widely used in the communication industry, and it has long been known that 180 degree phase shifts are optimal for transmission of a string of 1""s and 0""s. Typically, an xe2x80x9cexciterxe2x80x9d is used to generate a low level phase modulated signal, which is then amplified by means of a power amplifier. Resonant circuits are often used, as the signal bandwidth is usually small compared to the carrier frequency. This approach works well for high frequency signals where the signal bandwidth is small compared to the signal frequency. This conventional approach is also intended to drive resistive loads, such as a tuned antenna, which do not store energy. Due to the fact that power amplifiers are generally on the order of about 50% efficient, the efficiency of this approach is relatively low.
A magnetic field could be used to transmit information from downhole sensors if a magnetic dipole is located above the bit and the magnetic field is detected at the surface. Such a system would overcome the disadvantages of an electric dipole system, while potentially providing cost and reliability benefits over existing systems.
This application, theoretically operable bi-directionally and over distances of several kilometers, could be configured so that the transmitter would be controlled from the surface to conserve power. A combination battery/downhole power generator would add to the running time and enable higher output power levels.
The choice of transmission frequency of such a system would need to be a compromise between the need to keep the frequency low (in the 10 Hz range) to avoid the excitation of eddy currents in the formation, and the need to avoid the low frequency noise background of the earth""s magnetic field. The best approach to creating a large dipole downhole, would be to use a 10 meter drillpipe section as a support for the antenna. This section would be positioned at the bottom of the drill string above the steering sensors and the antenna would be excited by a large coil wrapped around the antenna section. Protection for the coil would be provided by an overall outer cover.
There are three special problems to be dealt with in the operation of a low frequency magnetic antenna: (1) the stored inductive energy in the antenna is very large and efficient operation is only possible if this energy is captured and reused; (2) the transmitted signal bandwidth is of the same order of magnitude as the frequency carrier, and simple resident circuits will not work; and (3) the frequency of this signal must be very tightly controlled, usually requiring that it be locked to a crystal oscillator. In addition to these problems, the electronics used for borehole telemetry must often operate at high temperature where the ability to dissipate heat is severely limited.
Accordingly, it one object of this invention to provide a magnetic transmission system which operates with minimal power consumption, that is, the achievement of phase modulation with only the minimal losses of power needed to overcome losses in the antenna itself.
It is another object of this invention to provide a magnetic transmission system which addresses the three special problems set forth hereinabove.
It is yet another object of this invention to provide a circuit which offers the possibility of much lower power dissipation than is possible by means of conventional circuitry.
It is another object of this invention to provide a circuit which captures the energy stored in the inductive load and reuses it in the next cycle instead of dissipating it.
These and other objects of this invention are addressed by a circuit for driving a current into an inductor, such as a magnetic antenna, comprising at least one main capacitor, a power supply operatively connected to the at least one main capacitor, the inductor, and at least two pairs of switches connecting the at least one main capacitor to the inductor, whereby the connection of the at least one main capacitor to the inductor is made in either polarity.
In accordance with the method for driving a current into an inductor in accordance with this invention, a current is applied to the main capacitor and a voltage is applied to the inductor from the main capacitor resulting in generation of inductive energy. At the end of a cycle, the inductive energy is recaptured by the main capacitor.